Prism system for refractometer of the abbe type



June 21, 1949. 2,474,044

PRISM SYSTEM FOR REFRACTOMETER OF THE ABBE TYPE .J. W. FORREST Filed Feb. 12, 1947.

I /l//IV 4/1111, F l G 2. 34

J.W. FORREST Zmventor (Ittorneg Patented June 21, 1949 UNITED STATES PATENT OFFICE PRISM SYSTEM FOR REEFRACTOMETER OF THE ABBE TYPE Application February 12, 1947, Serial No. 728,026

Claims. 1

The present invention relates to improvements in refractometers of the critical angle type wherein the refractive index of liquid specimens is determined from the angular position of a dividing line between light and dark portions in the field of the eyepiece. The sharpness of the dividing line is determined largely by the nearness of approach to the critical angle of total reflection at which incident light rays are directed upon the liquid specimen. A serious detriment to the sharpness of the dividing line in certain critical angle refractometers resides in the fact that when measuring the refractive index of liquids, parts of the casing for one of the refractive prisms obstruct light rays of truly grazing incidence, thus making it impossible to determine the real angle of total reflection with precision.

It is an object of this invention to provide in a refractometer means which permits light rays at truly grazing incidence to be projected upon the surface of the liquid specimen whereby the angle of total reflection can be determined with the same precision as in the case of a solid test specimen. Another object is to provide in a refractometer improved means for retaining the liquid specimen in contact with the refracting prism and directing light rays at grazing incidence to the surface of the specimen. A further object is to provide in a refractometer improved means for directing light rays upon a liquid specimen which is being measured.

The objects related above and others will be apparent to those skilled in this art by reference to the following description and accompanying drawing in which:

Fig. 1 is a sectional and partly diagrammatic view of a preferred form of my invention,

Fig. 2 is a sectional View taken on the line 2-2 of Fig. 1,

Fig. 3 is a diagrammatic View of the light reflective and refractive members comprising the liquid retaining means of my invention.

A preferred embodiment of my invention is shown in Figs. 1-3 of the drawing. It is to be understood that my invention relates to the prism unit and illuminating means associated therewith and that the invention may be used in the Abb type of refractometer such, for example, as that disclosed in U, S. Patent No. 2,080,841, issued to H. W. Straat. The construction and operation of the Abb refractometer is well known to those skilled in the art and in Fig. 1 I have shown, diagrammatically, how my invention is used in cooperation with the optical system of the Abb refractometer.

The structure of this type of instrument, which forms no part of the present invention and is well known in the art, embodies an alidade or arm I6 having an axis in common with the prism housing and a magnifier H, with reticle, mounted on the arm I6 for cooperation with the arcuate scale I8. A telescope I9 is in fixed relation to the scale l8 so that both may turn about the axis of the instrument for measurement of the critical angle in a manner well known in the art.

Instead of the usual prism housing and prism unit embodied in the Abb type of refractometer, I employ my novel prism unit such as is disclosed in Figs. 1 and 2. In this unit, a light refracting prism I 0 is secured by cement II within the housing I2. The prism I0 is provided with the polished exit face I5 and the entrance face I3 against which is held the liquid specimen I4 which is being tested.

According to my invention, the liquid specimen I4 is held in a shallow chamber formed by the cooperative relation of the entrance face I3 on one side and an indented or recessed refracting surface 2I on the opposite side extending along the adjacent side of a block 22 of light-transmitting material such as glass. In one successful form of my invention the chamber 2!! has a depth of about 0.1 mm. The surface 2|, which may be either ground or polished, is substantially parallel to the entrance face I3. If desired, ports 23 and 23' may be provided in the ends of the chamber 20 as shown in Fig. 1 for the circulation of the liquid specimen therethrough and a seal 24 may be inserted between the prism housing I2 and block 22 to prevent leakage of the liquid. The light transmitting block 22 is generally rectangular in form and, as shown in Fig. 2, is provided on either side of chamber 20 with polished contacting surfaces 25 and 25' which engage the entrance face I3. The block 22 is mounted within a housing 26 by means of cement 26'. The adjacent faces of housings I2 and 26 are held firmly together by means of screws I2 so that the liquid I4 is confined within chamber 20.

The salient feature of my invention resides in the means for directing light rays of truly grazing incidence upon the surface of the liquid specimen M where it contacts the recessed surface 2 i. This is accomplished by the inclined silvered surface 28 formed on the end of block 22 in alignment with an opening 21 in housing 25. The mirror surface 28 terminates sharply along the edge 29 at the recessed surface 2| and is disposed at such an angle as to direct light rays from a suitable light source 30 at truly grazing incidence upon the interface between the liquid specimen l4 and the surface 2|. Although the reflecting surface 28 is shown disposed at an angle of about 45 degrees to the surface 2|, it is obvious that other angular positions of the surface 28 and location of light source 30 could be used, the only requirement being that the light rays must be reflected substantially parallel to surface 2! so that some of the rays will strike the surface 2! at grazing incidence.

In the commonly used Abb typerefractometer of the prior art, the metal mounting parts which hold the prism prevent the light ray from striking the interface, between the liquid specimen and one prism, at a true grazing incidence. By contrast, the illuminating means of my invention shown in Fig. 3 illustrates how light rays incident on the reflecting surface 28 are reflected onto the surface of the liquid specimen [4 through the block 2'2 at truly grazing incidence and are refracted thereby through the specimen I4 and then through the prism ID to emerge from the exit face 15 and enter the telescope l9. The operation of a refractometer embodying the present invention is substantially the same as the operation of the well-known Abbe type refractometer.

Provided in the prism housing I2 and block housing 26, respectively, are the 'fluid chambers 33 and 34 through which a constant temperature fluid may be circulated to control the temperature of the liquid specimen M in the usual manner.

It will be seen that there is here provided a refractometer for testing liquids which is capable of measuring the true critical angle of reflection of the light rays incident on the specimen with precision in accordance with the objects stated heretofore. Various modifications can obviously be made without departing from the spirit of my invention as pointed out in the appended claims.

I'claim:

In a refractometer of the critical angle type for testing liquids comprising, a prism having a light refractive face against which a liquid speci- =men is held, a light transmitting member having .a surface which is parallel to and spaced from said refractive face to provide .a space for said liquid specimen, anda reflecting surface formed on said member at an angle to and terminating at said first named surface so as to direct light .rays from a light source through said member .at grazing incidence to the interface formed by the specimen and said surface so that said rays .are refracted through said liquid specimen and prism.

2. In a refractometer of the critical angle type for testing liquids comprising, a prism having a light refractive face against which a liquid specimen is held, a transparent blockin contact with said light refractive face and having a, recess along the adjacent side which forms together with said light refractive face aretaining chamber for said liquid specimen, said block having a reflecting surface thereon so disposed as to direct light rays from a source at grazing incidence along the recessedside of said chamber upon the 4. surface of said liquid specimen for refraction through said liquid specimen and prism in sequence.

3. In a refractometer of the critical angle type for testing liquids, a light refractive prism having an entrance face in contact with a liquid specimen, a block of light transmitting material having a recessed surface therein spaced from and substantially parallel to said refracting face to form therewith a chamber for retaining said liquid specimen, .a reflecting surface at one end of the block, said reflecting surface extending to said recessed surface and being positioned at such an angle thereto that light rays from a. light source may be projected at grazing incidence upon the interface formed by the specimen and said surface and be refracted by said specimen and prism in sequence.

4. In a refractometer for testing liquids, the combination of a prism having a refractive face, a light transmitting .member having a refractive surface which .is spaced from and substantially parallel to said refractivejfaceto ,provide a space therebetween for holding aliquid specimen, .and means for directing light .rays at grazing .incidence onto the interface formed by the liquid specimen and said surface, said means comprising a reflecting surface formed at one end of said member, said reflecting surf-ace terminating at the refractive surface and being angularlydisposed thereto so as to reflect light rays .from a light source .at grazing incidence onto said interface.

5.'In a refractometer for testingliquids, a prism having a polished entrance face, a prism housing, a block of light transmitting material provided with a recessedsurface which forms together with saidentrance face za'shallow chamber for retaining a liquidspecimen, -a reflecting surface on one end of said :block terminating sharply at said recessed surface and so disposed with reference to a light source as toreflect light rays therefrom upon the interface lformedby the specimen and recessed surface at grazing incidence to said interface-2. housing-forsaid block having an opening therethrough in alignment with said light source and zreflectingsurface, and means for holdingsaid block housing and prism housing together.

JOHN -W. FORREST.

REFERENCES .CIT-ED The following references are .of record .in the file of this patent:

UNITED STATES PATENTS Number Name Date I 1,471,342 Logan Oct. 23, :1923 1,868,908 Lindley July 26,, .1932 2,065,114 Cahn et 'al Dec. 22, 1936 2,240,988 Hertel May 6, 194 1 FOREIGN PATENTS Number Country Date 469,957 Germany .Jan. .3, 1929 

